1. Field of the Invention
The present invention relates to a swash-plate hydraulic pressure device such as a hydraulic pump or a hydraulic motor, and more particularly to a swash-plate hydraulic pressure device having improved plungers and swash plate.
2. Description of the Prior Art
One known swash-plate hydraulic pressure device is disclosed in Japanese patent application No. 3-199621 for example. The swash-plate hydraulic pressure device, which may be used as a hydraulic pump or a hydraulic motor includes a cylinder block having an annular array of cylinder holes defined therein around an axis and held in communication with an oil passage, and a plurality of plungers reciprocally movably disposed in the cylinder holes, respectively. A swash plate rotatable relatively to the cylinder block is disposed in surrounding relation to the cylinder block. The swash plate has an annular array of partly spherical recesses defined therein and held in abutment against respective partly spherical tip ends of the plungers.
When the swash plate is rotated with respect to the cylinder block, the plungers are caused to move reciprocally in the respective cylinder holes for drawing and discharging working oil into and out of the cylinder holes. At this time, the swash-plate hydraulic pressure device operates as a hydraulic pump. Alternatively, working oil is introduced into and discharged out of the cylinder holes to move the plungers reciprocally in the respective cylinder holes, forcing the swash plate to rotate with respect to the cylinder block. At this time, the swash-plate hydraulic pressure device operates as a hydraulic motor.
Heretofore, both the plungers and the swash plate have been made of steel. Under rigorous operating conditions, e.g., when the swash-plate hydraulic pressure device rotates at a high speed or under a high hydraulic pressure, however, the steel plungers tend to wear rapidly.
Japanese laid-open patent publication No. 62-104616 discloses a guide roller for rolling a wire rod of metal, which is an entirely unrelated art but the disclosed guide roller is made of a ceramic material which is highly resistant to wear and heat, and highly lubricatable. Specifically, the disclosed guide roller is made of or surfaced with silicon nitride having a porosity of 1% or less and a surface roughness of 6 s or lower.
However, the specifications of the ceramic material disclosed in that Japanese patent publication cannot be directly applied for use on the plungers of the swashplate hydraulic pressure devices. Specifically, since the plungers are subject to a much higher pressure than the disclosed guide roller, if the maximum surface roughness (Rmax) of the plungers was 6 s, then the oil film would be broken between the tip ends of the plungers and the swash plate, resulting in direct contact between the plungers and the swash plate and hence a localized increase in the pressure between the plungers and the swash plate. Further, the porosity of a ceramic material, which is the ratio of the volume of pores of the material to the volume of the material, is not necessarily an exact representation of the conditions of the contacting surfaces of the plungers and the swash plate.
Another problem is that because the elastic coefficient and coefficient of friction of the ceramic material is different from those of the steel, ceramic plungers would suffer abnormal wear and seizure and be liable to jump out of the recesses of the swash plate in high-speed and high-pressure operation if the ceramic plungers were of the same dimensions and shape as the steel plungers.